People throughout the Monadnock region were stunned and saddened Monday after learning of the deaths of Rick and Duffy Monahon, the husband-and-wife architects well known for their interest in historical preservation and land conservation and their active involvement on Peterborough boards and committees.

“Right now, our community is reeling,” Peterborough Town Administrator Pam Brenner said Monday morning, as she sat in her office across the street from the building where the Monahons’ small architectural firm is located. “Rick was chair of our Planning Board, involved in the renovation of our community center. Hardly a day would go by that I wouldn’t see [Duffy] on the street, always with a pile of papers, always in a hurry. ... They will leave a vacuum that the town will be hard pressed to fill.”

Richard and Mary Monahon, who were almost universally known as Rick and Duffy, died after a car crash on Routes 9 and 202 in Hillsborough late Sunday afternoon, as they were returning home to Peterborough after a day of skiing at Mount Sunapee.

According to Hillborough police, Richard Monahon, 69, was driving a 2013 Subaru Forester westbound with his wife, 71, as a passenger at 5:09 p.m. The Forester collided with an eastbound 2005 Toyota Camry driven by Donald Robb, 68, of Concord, with his wife, Brenda Mary Robb, 62, of Concord as a passenger.

According to police, the Monahons died at the scene. Donald Robb was also pronounced dead at the scene, while Brenda Mary Robb was rushed to Concord Hospital and then airlifted to a hospital in Boston, where she later died of injuries.

Police are investigating which car crossed the dividing line and circumstances that may have contributed to the crash.

Initial investigations indicate that alcohol was not a factor.

The roadway was shut down for about six hours following the crash.

Joslin Kimball Frank of Keene, a friend who had been skiing with the Monahons, was driving immediately behind them when the accident occurred.

“It had been one of those gorgeous days,” Frank said in a phone conversation on Monday. “We’d talked on the chair lifts and skied all day. When we finished, we went inside for tea and Rick described his last run. ‘I haven’t skied better since I can’t remember when,’ he said. He was ecstatic.”

Frank said she was following the Monahons’ car, en route to their home in Peterborough where the skiers were planning to have dinner, when the accident happened as the Subaru was about to turn off Route 9 at the exit where Route 202 turns south toward Peterborough.

“Everything was fine,” Frank said. “The road was dry. It was light outside. It appeared his car may have drifted over into the other lane. It all happened so quickly. I think a blinker went on. Then there was an explosion as these cars hit. It was just awful.”

Frank said rescuers arrived quickly and moved the Monahons into ambulances.

“They did not respond,” she said. “It was clear that it was really serious.”

Frank described the Monahons as “people of tremendous energy.”

“They had probably the widest circle of friends of anyone I know,” Frank said. “They had such a life force. They were always willing to try anything. They made life so much fun.”

The Monahon’s daughter, Sophie, is in graduate school at the University of Texas at Austin. Duffy Monahon also had children from an earlier marriage.

The Robbs were well-known in Concord, where they lived for 40 years. Donald Robb retired as a prison corrections officer in 1999 and was active in local adult softball leagues. Brenda Mary Robb retired from teaching as a junior high teacher in Concord in 1998 and was active in the city. She was currently a member of the city’s Board of Ethics. They have two children, both of whom are married, and two grandchildren.

Their son, Michael Robb, 40, of Concord, told the Concord Monitor, “It’s not lost on us that another set of people are feeling the way we are feeling. There is a whole other family going through what we are going through. We are only half the people that are suffering.”

Anyone with information about the accident is asked to contact Hillsboro Police at 464-5512.